Stents are endoprostheses which can be deployed into the lumen of an artery or vein, a common bile duct, the urethra or other body passageway. Stents may be employed in such passageways for many purposes, including expansion of a lumen, maintenance of the lumen after expansion, and repair of a damaged intima or wall surrounding a lumen. With respect to arteries, for example, stents may be used as, or in conjunction with, intralumenal grafts in the maintenance of patency of a lumen following angioplasty. In such cases a stent may be used to prevent restenosis of the dilated vessel, to prevent elastic recoil of the vessel, or to eliminate the danger of occlusion caused by "flaps" resulting from intimal tears associated with the angioplasty. In other instances, stents may be used to treat aneurysm, tears, dissections and other continuity faults, as, for example, in the splenic, carotid, iliac and popliteal vessels. By way of further example, it is known to use a stent to maintain the patency of a urethra compressed by an enlarged prostate gland.
In one class of expandable stents commonly referred to as "rolled" stents, a sheeted material is rolled onto the outer distal circumference of a support member or "core". The sheeted material is then positioned at a targeted treatment area and expanded. Rolled stents can be characterized according to: (1) the method by which the rolled sheet is maintained in a compressed configuration; and (2) the method by which the sheet is expanded.
Lane, Self Expanding Vascular Endoprosthesis for Aneurysms, U.S. Pat. No. 5,405,379 (Apr. 11, 1995) describes a stent which employs a self expanding sheet. The sheet is forcibly rolled into a compressed configuration, and then inserted into a catheter to maintain the compressed configuration. Expansion takes place by ejecting the sheet from the end of the catheter.
Kreamer, Intraluminal Graft, U.S. Pat. No. 4,740,207 (Apr. 26, 1988) describes a rolled stent in which a sheet of stainless steel is rolled around an angioplasty type balloon. After being introduced into a treatment area, the sheet is expanded by inflating the balloon with a fluid. In this case compression is maintained during the early stages of deployment by the relaxed nature of the sheet in the compressed configuration, i.e., the internal mechanical resistance of the sheet to deformation. Expansion of the sheet, on the other hand, occurs under radial pressure exerted by the expanding balloon.
Sigwart, Intravascular Stent, U.S. Pat. No. 5,443,500 (Aug. 22, 1995) describes a stent in which a flat sheet is perforated to form a reticulated or lattice type structure having a ratcheting locking mechanism. Compression in stents according to the Sigwart patent are maintained by a holding wire or adhesive, and the sheet is contemplated to be expanded under the influence of an angioplasty balloon.
Sigwart also describes another stent comprising an elastic stainless steel mesh. The diameter of the mesh is slightly larger than the normal inner diameter of the vessel to be treated, so that the mesh can exert a residual radial pressure on the arterial wall after being implanted. Before being introduced into a patient's blood vessel the stent is reduced in diameter. The reduced diameter is maintained while advancing the stent into a target treatment area by an outer sleeve. Once the device is implanted, the stent is deployed by withdrawal of the outer sleeve. In this instance, compression is thus maintained by the outer sleeve, and expansion is achieved by removal of the outer sleeve.
Alfidi and Cross, Vessel Implantable Appliance and Method of Implanting It, U.S. Pat. No. 3,868,956 (Mar. 4, 1975) describes a stent which utilizes a recovery alloy such as nitinol. In such stents an initial expanded configuration is permanently set into the alloy by heating the material to a relatively high temperature while the alloy is maintained in the expanded configuration. The alloy is then cooled and deformed to a compressed configuration. The compressed configuration is retained at room temperature, but recovers to the expanded configuration when reheated to a transition temperature. Here, compression is maintained during the early stages of deployment by the internal mechanical resistance of the alloy against deformation, and the sheet is expanded under the influence of heat.
These and all other known teachings reflect the accepted wisdom that rolled stents are to be maintained in their compressed configurations by the operation of static forces (e.g., biasing produced by the internal mechanical resistance of the sheet to deformation, presence of holding wires, outer sleeves and so forth), while expansion of the sheeted materials is to be produced by application of a dynamic force (e.g., radial pressure exerted by an expanding balloon, application of heat, removal of a holding wire or sheath, and so forth). While such strategies undoubtedly have their benefits, it is useful to have stents which operate outside of these accepted constraints.
Where the stent is to be deployed in very small vessels of the body, such as the arteries in the brain, the size of the stents is quite small, and the material used for the stents is on the order of 0.0001-0.0004 inches thick. The small size and extreme thinness of the stent material makes it difficult to deploy the stent using the typical push-pull type deployment mechanisms generally used for stents. The frictional force exerted on the stent by the catheter sheaths and cores as they slide over the stent often tears the stent. In our co-pending U.S. Pat. application Ser. No. 08/762,110, filed Dec. 9, 1996, we provide a number of devices that do not require any sliding movement of the stent or catheter sheath relative to each other. The devices described below provide additional mechanisms and methods for deploying stents while minimizing the frictional forces operating between the stents and the catheters used for their insertion.